<Window x:Class="WpfApplication1.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" Height="100" Width="300">
    <Grid>
        <ComboBox VerticalAlignment="Top" HorizontalAlignment="Left"
                  Width="100" IsSynchronizedWithCurrentItem="True" >
            <ComboBoxItem Content="item 1"/>
            <ComboBoxItem Content="item 2"/>
            <ComboBoxItem Content="item 3"/>
        </ComboBox>
        <Button Content="Test" VerticalAlignment="Bottom" HorizontalAlignment="Right"
                Width="100" Click="Button_Click" />
    </Grid>
</Window>

private void Button_Click(object sender, RoutedEventArgs e)
{
    MessageBox.Show("Button clicked");
}

MyComboBox.AddHandler(
    UIElement.MouseLeftButtonDownEvent,
    new MouseButtonEventHandler(
        (sender, e) =>
        {
            MyButton.Command.Execute(null);
        }),
    handledEventsToo: true
);